A study in the Waikato has found that traumatic brain injuries (TBI) are far more common than previously thought.

Findings reveal that rates of TBI in New Zealand, at 790 per 100,000 people per year, are far higher than so far found in other developed countries.

While 95 per cent of all cases were found to be mild TBI, research team leader Professor Valery Feigin warned that mild cases often related in significant health problems later in life.

In a study area with a population of about 173,000, 1369 people were found to have TBI in the year to the end of February 2011. Out of that total, 71 cases were classed as moderate to severe.

Based on the research, Feigin, from AUT's national institute for stroke and applied neuroscience, estimated more than 36,000 new TBIs were happening in New Zealand each year.

Previous statistics had grossly underestimated the extent of TBI in this country, he said.

"This means one new TBI happens every 15 minutes, far more than the number of new heart attacks and greater than five times the number of new strokes."

A TBI happens when an external force, such as a bump or blow to the head disrupts the normal function of the brain.

Falls accounted for 37.7 per cent of the injuries in the study. Those included some sports-related incidents such as falling from horses, or falling from a height when on a tree or a ladder, Feigin said.

TBIs resulting from falls were proportionally more common among young children and people aged over 65.

Among the older group those were more likely to be due to slips and stumbles, while among youngsters they were more likely to be falls from heights or during physical play.

Mechanical forces - when head trauma is caused by an animal, inanimate object, or due to accidental force from another person - accounted for 21 per cent of the cases.

The category included contact sports which were responsible for about 5 per cent of the total cases.

Transport accidents caused 20.2 per cent of the cases, while 16.7 per cent were from assaults.

The rate of 790 cases per 100,000 people compared with 47 to 453 found in studies in Europe, and 51 to 618 cases in North America.

More research needed to be done to work out why the incidence of TBI was so much higher in this country, Feigin said.

He acknowledged part of the reason could be the way the study was carried out, but said part of the reason could also be that the number of injuries from assaults were almost twice the level found overseas.

The study, published today in the medical journal The Lancet, is believed to be the first large population-based study to investigate TBI incidence across the spectrum of severity in all age groups in a geographical region with urban and rural populations.

Feigin said mild TBI could result in such immediate health problems as a relatively short loss of memory of the event or what happened just afterwards, and a very minor loss of consciousness at the time of the injury.

They could lead to significant health problems later in life, progressing even to dementia, and there was also a stroke link to mild TBI.

Social impacts could include changes in a person's character, and could be associated with depression. There could be behavioural changes, with people starting to drink heavily or becoming aggressive.

"It's a huge problem," Feigin said.

"The consequences of mild TBI are certainly not mild."

But if people with mild TBI were treated in a timely manner then many of the consequences could be avoided.

The Waikato study found males had a 77 per cent greater risk of TBIs than females, and that Maori had a 23 per cent greater risk than New Zealand Europeans.

The risk of moderate to severe TBI was almost 2.5 times greater in the rural than the urban population. That difference was driven largely by a higher incidence rate of TBI due to transport accidents among rural people.